1. Field of the Invention
The present invention relates to a flame retardant composition. More particularly, this invention relates to a polypropylene composition which contains a halogenated aliphatic bisimide and 2,3-dimethyl-2,3-diphenylbutane.
2. Description of the Prior Art
A variety of compounds are known which impart satisfactory flame resistance, smoke suppression and self-extinguishing properties to thermoplastic compositions.
A brominated cyclic bisimide which is a recognized flame retardant additive in polypropylene is N,N'-1,2-ethane-bis(5,6-dibromonorbornane-2,3-dicarboximide). Wolford et al, U.S. Pat. No. 3,917,642, teach the use of novel compounds useful as flame retardants in normally flammable organic polymers. N,N'-1,2-ethane-bis(5,6-dibromonorbornane-2,3-dicarboximide) is specifically claimed. Dotson et al, U.S. Pat. No. 3,923,734, claim the use of brominated cyclic bisimides in polymer compositions. Dotson et al, U.S. Pat. No. 3,786,023, teach the flame retarding of polypropylene with N,N'-hexane-1,6-bis(5,6-dibromonorbornane-2,3-dicarboximide) and a synergist. The synergist may be a metal oxide or a phosphite.
A variety of flame retardant synergists are frequently used in polymer compositions. The use of 2,3-dimethyl-2,3-diphenylbutane as a synergist in certain polymer compositions is known. Weber et al, U.S. Pat. No. 3,420,786, teach the flame retarding of polystyrene foam with aliphatic bromine containing compounds such as hexabromocyclododecane and 2,3-dimethyl-2,3-diphenylbutane. These particular bromine containing compounds are effective flame retardants but are thermally unstable. French Demande 2,017,234 teaches flame retardant polypropylene containing these same compounds. Underwood et al, U.S. Pat. No. 3,850,882 teach the flame retarding of propylene with aromatic bromine compounds and 2,3-dimethyl-2,3-diphenylbutane. These aromatic bromine compounds are thermally stable, however, they are less efficient flame retardants than aliphatic bromine compounds.
Canadian Pat. No. 919,856 teaches a flame retardant additive system which consists of compounds such as 2,3-dimethyl-2,3-diphenylbutane and halogen-containing compounds having chlorine or bromine in the connecting bridge between two cycloalkyl groups.
It has now been discovered that 2,3-dimethyl-2,3-diphenylbutane may be used in polypropylene compositions along with halogenated aliphatic bisimides as described in this invention to improve the flame-retarding of the polypropylene composition. The halogenated aliphatic bisimides described in this invention are efficient flame retardants and thermally stable. By "thermally stable" it is meant that the compound does not degrade appreciably under normal processing conditions. If a compound does degrade under processing conditions then a toxic and corrosive gas is generated (hydrogen bromide) and the flame retardant properties of the compositions are decreased. The halogenated aliphatic bisimide flame retardant has low volatility, is stable at polypropylene processing temperatures, is readily dispersible in polypropylene, is compatible with polypropylene, and is effective in sufficiently low concentrations to minimize the degradation of physical properties of the polypropylene.